In a two-layer tread consisting of a cap tread and a base tread, the base tread often contains carbon black alone as filler for durability, rigidity, conductivity, and other properties, while the recent cap tread often contains a large amount of silica (e.g. at least 50% by mass of the total filler) to improve fuel economy and wet grip performance.
Such a two-layer tread with a base tread compound containing carbon black alone as filler and a cap tread compound having a high silica content (silica-rich compound) usually has a large difference in rubber physical properties between the two layers, causing problems such as poor handling stability and road noise generation.
These problems are more significant particularly in cold weather tires including a cap tread with low hardness because of a larger difference between the cap tread and the base tread with high rigidity. Moreover, due to the recent improvement of road conditions, more and more vehicles with cold weather tires are driven at high speeds. However, since conventional cold weather tires place an importance on the performance on ice and snow achieved during running at relatively low speeds, they tend to show inferior handling stability and grip performance during high speed running.
Moreover, foamed treads have lower rubber density and better performance on ice and road noise performance as compared to non-foamed rubbers. However, they have largely different physical properties from base tread rubbers having relatively high rigidity and high rubber density. Furthermore, the incorporation of a large amount of silica can lead to problems with tire conductivity. For example, although Patent Literature 1 proposes the use of a base tread formed of a conductive material, there is a problematic difference in physical properties between the base tread containing a large amount of carbon black and the cap tread.
Thus, a need exists for a technique to improve high speed performance (handling stability, grip performance) on snow- and ice-free cold roads and road noise performance while ensuring performance on ice and snow, conductivity, and other properties, thereby achieving a balanced improvement in properties including fuel economy, performance on ice and snow, high speed performance on snow- and ice-free cold roads, road noise performance, and conductivity.